A cationic lipid is an amphiphilic molecule having a lipophilic region containing one or more hydrocarbon groups and a hydrophilic region containing at least one positively charged polar head group. The formation of a complex which is positively charged as a whole between a cationic lipid and a macromolecule such as a nucleic acid facilitates the entry of the macromolecule such as a nucleic acid into a cytoplasm through a cell plasma membrane, and therefore, the cationic lipid is useful. This process, which can be performed in vitro and in vivo, is known as transfection.
Patent Documents 1 and 2 disclose cationic lipids and lipid particles containing the lipid, which are advantageous for in vivo delivery of a nucleic acid into a cell and for use in a nucleic acid-lipid particle composition suitable for therapy of diseases. For example, Patent Document 1 discloses a cationic lipid such as
    2,2-dilinoleyl-4-(2-dimethylaminoethyl)[1,3]-dioxolane (DLin-KC2-DMA); and for example, Patent Document 2 discloses a cationic lipid such as
    (6Z,9Z,28Z,31Z)-heptatriaconta-6,9,28,31-tetraen-19-yl 4-(dimethylamino)butanoate (DLin-MC3-DMA).